Conventional acoustic apparatus are provided with bass control and treble control knobs on the operation panel which are manipulated for adjustment to realize the desired tone.
FIG. 6 shows a tone control circuit heretofore proposed for effecting tone control in this manner (Unexamined Japanese Pat. Publication No. SHO 59-212012).
With this circuit, the signal applied to an input terminal 100 is fed to a low-pass filter 101, a band-pass filter 102 and a high-pass filter 103. The output signals from the low-pass filter 101 and the high-pass filter 103 are passed through variable gain amplifiers 104 and 105, respectively, and fed to an adder 106 along with the output signal from the band-pass filter 102. The audio signal obtained at an output terminal 107 is sent to a loudspeaker via a power amplifier.
A bass control signal is applied to a control signal input terminal 108 to adjust the gain of the amplifier 104 for bass control, while a treble control signal is applied to another control signal input terminal 109 to control the gain of the amplifier 105 for treble control.
To reproduce an acoustically dynamic sound, it is generally effective to boost the low-frequency component of the audio signal as the level of the signal lowers. With the conventional circuit of FIG. 6, however, the bass control signal and the treble control signal are prepared by adjusting the corresponding knobs on the operation panel, so that every time the level of the audio signal varies, there arises a need for tone adjustment to realize optimum frequency characteristics, thus entailing the problem of a cumbersome adjustment procedure.
The tone control circuit of FIG. 6 includes three filters, i.e., the low-pass filter 101, band-pass filter 102 and high-pass filter 103 which require capacitors of large capacity. This presents extreme difficulties in incorporating the three filters 101, 102 and 103 into the tone control circuit when the circuit is to be fabricated in the form of an IC, consequently offsetting the advantage to be obtained by constructing the circuit as an IC.